The collector plate absorbs as much of the irradiation as possible through the glazing, while losing as little heat as possible upward to the atmosphere and downward through the back of the casing. The collector plates transfer the retained heat to the transport fluid. To maximize the energy collection, the absorber of a collector should have a coating that has high absorptance for solar radiation (short wavelength) and a low emittance for re-radiation (long wavelength). Such a surface is referred as a selective surface. The absorptance of the collector surface for shortwave solar radiation depends on the nature and color of the coating and on the incident angle. Usually black color is used, but various color coatings have been proposed by Tripanagnostopoulos et al. (2000); Wazwaz et al. (2002); and Orel et al. (2002), mainly for aesthetic reasons.
By suitable electrolytic or chemical treatment, surfaces can be produced with high values of solar radiation absorptance (a) and low values of longwave emittance (e). Essentially, typical selective surfaces consist of a thin upper layer, which is highly absorbent to shortwave solar radiation but relatively transparent to longwave thermal radiation, deposited on a surface that has a high reflectance and low emittance for longwave radiation. Selective surfaces are particularly important when the collector surface temperature is much higher than the ambient air temperature. The cheapest absorber coating is matte black paint; however, this is not selective, and the performance of a collector produced in this way is low, especially for operating temperatures more than 40°C above ambient.
An energy-efficient solar collector should absorb incident solar radiation, convert it to thermal energy, and deliver the thermal energy to a heat transfer medium with minimum losses at each step. It is possible to use several design principles and physical mechanisms to create a selective solar-absorbing surface. Solar absorbers referred to as tandem absorbers, are based on two layers with different optical properties. A semiconducting or dielectric coating with high solar absorptance and high infrared transmittance on top of a non-selective, highly reflecting material such as metal constitutes one type of tandem absorber. Another alternative is to coat a non-selective, highly absorbing material with a heat mirror that has a high solar transmittance and high infrared reflectance (Wackelgard et al., 2001).
Today, commercial solar absorbers are made by electroplating, anodiza-tion, evaporation, sputtering, and applying solar selective paints. Of the many types of selective coatings developed, the most widely used is black chrome.
Much of the progress in recent years has been based on the implementation of vacuum techniques for the production of fin-type absorbers used in low-temperature applications. The chemical and electrochemical processes used for their commercialization were readily taken over from the metal finishing industry. The requirements of solar absorbers used in high-temperature applications, however—namely, extremely low thermal emittance and high temperature stability—were difficult to fulfill with conventional wet processes. Therefore, large-scale sputter deposition was developed in the late 1970s. Nowadays, the vacuum techniques are mature, are characterized by low cost, and have the advantage of being less environmentally polluting than the wet processes.
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Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.